1. Field of the Invention
The present invention relates to a plasma CVD apparatus and a plasma processing method. More particularly, the present invention relates to a plasma CVD apparatus and a plasma processing method which are suited for the production of various electronic devices such as semiconductor devices, electrophotographic photosensitive devices (or electrophographic light receiving members), image input line sensors, flat panel displays, image pickup devices, photovoltaic devices, and the like.
2. Related Background Art
Recently, in the production of an electronic device such as semiconductor device and the like, a plasma CVD apparatus has been often used. Particularly, a plasma CVD apparatus in which a high frequency with 13.56 HMz or a microwave with 2.45 GHz is used has been widely used, since various substrates and depositing materials regardless of their properties being electrically conductive or electrically insulating can be optionally plasma-processed by the plasma CVD apparatus.
As an example of such plasma CVD apparatus, there can be mentioned a parallel plane plate type plasma CVD apparatus in which a high frequency energy is used, as shown in FIG. 1.
Description will be made of the plasma CVD apparatus shown in FIG. 1.
The plasma CVD apparatus shown in FIG. 1 comprises a reaction chamber 1 in which a cathode electrode 3 is arranged through a cathode electrode support 2. Around the cathode electrode 3, an earth shield 4 is arranged so as to prevent discharge from being generated between a side portion of the cathode electrode 3 and the reaction chamber 1. The cathode electrode 3 is electrically connected to a high frequency power source 10 through a matching circuit 9.
Reference numeral 5 indicates a counter electrode which is arranged in parallel to the cathode electrode 3.
Reference numeral 6 indicates a plate-like shaped substrate as an object to be plasma-processed which is positioned on the counter electrode 5. Reference numeral 7 indicates an exhausting means (such as vacuuming pump) which is communicated with the inside of the reaction chamber 1 through an exhaust pipe. Reference numeral 8 indicates a raw material gas supply source which is communicated with the inside of the reaction chamber 1 through a gas feed pipe.
The substrate 6 can be maintained at a desired temperature by means of a substrate temperature control means (not shown).
Plasma CVD (plasma chemical vapor deposition) using the plasma CVD apparatus shown in FIG. 1 is conducted, for example, in the following manner.
The reaction chamber 1 is evacuated to bring the inside to a desired high vacuum by operating the exhausting means 7, followed by introducing a given raw material gas from the raw material gas supply source 8 into the reaction chamber 1, and the gas pressure in the reaction chamber is maintained at a predetermined pressure. Thereafter, a high frequency power from the high frequency power source 10 is supplied to the cathode electrode 3, whereby plasma is generated between the cathode electrode 3 and the counter electrode 5, where the raw material gas introduced into the reaction chamber 1 is decomposed and excited to cause the formation of a deposited film on the substrate 6.
In this case, as the high frequency power, there is generally used an RF power with 13.56 MHz. In the case of using a discharging frequency of 13.56 MHz, although there are advantages such that the discharging conditions can be relatively easily controlled and a deposited film having excellent film quality can be obtained, there are drawbacks such that the raw material gas utilization efficiency is insufficient and the deposition rate for a film deposited is relatively small.
In view of such situation as above described, various studies have been made of a plasma CVD method using a high frequency with 25 to 150 MHz.
For instance, Plasma Chemistry and Plasma Processing, Vol. 7. No. 3. pp. 267-273 (1987) (hereinafter, referred to as Document 1) discloses a manner in which using a parallel plane plate type glow discharge decomposition apparatus, a raw material gas (silane gas) is decomposed with a high frequency energy having a frequency with 25 MHz to 150 MHz to form an amorphous silicon (hereinafter referred to as a-Si) deposited film on a substrate. Specifically, Document 1 describes that a-Si deposited films were formed while changing the frequency in the range of 25 MHz to 150 MHz; and the deposition rate in the case of using 75 MHz was 2.1 nm/sec which is the largest among others and which is greater by about five to eight times over that in the case of the plasma CVD using a frequency with 13.56 MHz. Document 1 also describes that the defect density, optical bandgap and conductivity of an a-Si film obtained are less influenced by an excitation frequency employed.
However, the film formation described in the Document 1 is of a laboratory scale. Document 1 does not even suggest anything of whether or not such effect as above described can be expected in the case of forming an a-Si deposited film having a large area. Further, Document 1 is absolutely silent about a manner of simultaneously forming an a-Si deposited film on a plurality of large area substrates to efficiently produce a plurality of large area semiconductor devices which can be desirably used in practice. In fact, Document 1 merely mentions a possibility that the use of higher frequencies (13.56 up to xcx9c200 MHz) opens interesting perspectives for fast processing of low cost, large area a-Si thin film devices in which thicknesses of several xcexcm are required.
The above example is of the case where the plasma CVD apparatus which is appropriate for plasma-processing a plate-shaped substrate.
Besides, an example of a plasma CVD apparatus which is appropriate for forming a deposited film on a plurality of cylindrical substrates is disclosed in European Patent Publication No. 154160 A (hereinafter referred to as Document 2). Particularly, Document 2 discloses a plasma CVD apparatus using a microwave power source having a frequency with 2.45 GHz (this apparatus will be hereinafter referred to as microwave plasma CVD apparatus) and a plasma CVD apparatus using a radio frequency (RF) power source (this apparatus will be hereinafter referred to as RF plasma CVD apparatus).
In the microwave plasma CVD apparatus disclosed in Document 2, since a microwave energy is used, the density of plasma generated upon film formation is extremely high. Because of this, a film-forming raw material gas is rapidly is decomposed, whereby the deposition of a film is conducted at a high speed. However, there is a problem such that it is difficult to stably and continuously form a high quality dense film.
Next, description will be made of the RF plasma CVD apparatus described in Document 2 while referring to FIGS. 2(A) and 2(B).
FIG. 2(A) is a schematic diagram illustrating an RF plasma CVD apparatus based on the RF plasma CVD apparatus described in Document 2. FIG. 2(B) is a schematic cross-sectional view, taken along the line Xxe2x80x94X in FIG. 2(A).
The RF plasma CVD apparatus shown in FIGS. 2(A) and 2(B) comprises a reaction chamber 100 in which six cylindrical substrate holders 105A each having a cylindrical substrate 106 for film formation positioned thereon are concentrically and spacedly arranged at a predetermined interval. The reaction chamber 100 has a plasma generation region A circumscribed by the substrate holders 105A. Reference numeral 105B indicates a dummy holder which serves to cap an end portion of the cylindrical substrate 106 positioned on the substrate holder 105A.
Each substrate holder 105A is provided with a heater 104 in the inside thereof so that the cylindrical substrate 106 can be heated from the inner side thereof. Each substrate holder 105A is held on a shaft 131 coupled to a driving motor 132 so that the substrate holder 105A can be rotated.
Reference numeral 103 indicates a cathode electrode arranged at a central position in the plasma generation region A. The cathode electrode 103 is electrically connected to an RF power source 111 through a matching circuit 109. The cathode electrode 103 serves to supply an RF power from the RF power source 111 in the plasma generation region A.
Reference numeral 130 indicates a support member by which the cathode electrode 103 is supported.
Reference numeral 107 indicates an exhaust pipe provided with an exhaust valve. The exhaust pipe 107 is communicated with an exhausting mechanism 135 provided with a vacuum pump (not shown).
Reference numeral 108 indicates a raw material gas supply system comprising gas reservoirs, mass flow controllers, valves and the like. The raw material gas supply system 108 is connected to a gas feed pipe 116 provided with a plurality of gas discharge ports through a gas supply pipe 117. Reference numeral 133 indicates a seal member.
The plasma CVD using the above RF plasma CVD apparatus is conducted, for example, in the following manner.
The reaction chamber 100 is evacuated to bring the inside to a desired high vacuum by operating the exhausting mechanism 135, followed by introducing a given raw material gas from the raw material gas supply system 108 into the reaction chamber 100 through the gas supply pipe 117 and the gas feed pipe 116, and the gas pressure in the reaction chamber 100 is maintained at a predetermined pressure. Thereafter, a given high frequency power from the RF power source 111 is supplied to the cathode electrode 103 through the matching circuit 109, whereby plasma is generated between the cathode electrode 103 and the cylindrical substrates 106 in the plasma generation region A, where the raw material gas introduced into the reaction chamber 100 is decomposed and excited to cause the formation of a deposited film on each of the cylindrical substrates 106.
In the case of using the RF plasma CVD apparatus shown in FIGS. 2(A) and 2(B), since the discharge space (comprising the plasma generation region A) is circumscribed by the cylindrical substrates 106, there is an advantage such that the raw material gas can be utilized at a high utilization efficiency. However, there are disadvantages such that in order to evenly form a deposited film on the entire surface of each of the cylindrical substrates, it is necessary to rotate the cylindrical substrates, and by rotating the cylindrical substrates, the substantial deposition rate is reduced to about ⅓ to ⅕ of that in the case of using the foregoing parallel plane plate type plasma CVD apparatus. This situation is due to the reason that as the discharge space is circumscribed by the cylindrical substrates, film deposition on a surface area of the cylindrical substrate which is positioned to oppose the cathode electrode is conducted at a deposition rate similar to that in the case using the parallel plane plate type plasma CVD apparatus, but on the remaining surface areas of the cylindrical substrate which are positioned not to oppose the cathode electrode, film deposition is slightly conducted.
Incidentally, using the RF plasma CVD apparatus shown in FIGS. 2(A) and 2(B) in which a cylindrical substrate is positioned on each of the substrate holders, the present inventors conducted the formation of an a-Si film on the entire surface of each of the cylindrical substrates while rotating the cylindrical substrates, where an RF energy having a frequency with 13.56 MHz (which is usually used in ordinary RF plasma CVD) was used as the discharge energy, SiH4 was used as the film-forming raw material gas, and the gas pressure in the reaction chamber upon the film formation was made to be several hundreds mTorrs under which the film deposition rate is increased but a powder of polysilane or the like is liable to occur. And based on the resultants, deposition rate was examined. As a result, the substantial deposition rate was found to be at most 0.5 nm/sec.
Now, in the case of producing an electrophotographic light receiving member having a photoconductive layer comprising an a-Si film using the RF plasma CVD apparatus shown in FIGS. 2(A) and 2(B), the a-Si film as the photoconductive layer is necessary to have a thickness of about 30 xcexcm. In order to form the a-Si film having such large thickness, it takes more than 16 hours at the foregoing deposition rate of about 0.5 nm/sec. Therefore, the productivity is unsatisfactory.
In addition, for the RF plasma CVD apparatus shown in FIGS. 2(A) and 2(B), when an RF energy having a frequency with 30 MHz or more is used as the discharge energy, uneven plasma is liable to form in the axial direction of the cylindrical substrate. Because of this, it is difficult to form a homogeneous deposited film having a uniform thickness over the entire surface of the cylindrical substrate.
By the way, in the image-forming industrial field, for the photoconductive material to constitute a light receiving layer in an electrophotographic light receiving member, it is required to be highly sensitive, to have a high S/N ratio (photo-current (Ip)/dark current (Id)), to have absorption spectrum characteristics suited for an electromagnetic wave to be irradiated, to be quickly responsive, and to have a desired dark resistance. Besides, it is also required to be not harmful to living things, especially human body, upon use.
Particularly for electrophotographic light receiving members used in an electrophotographic apparatus which is used as a business machine at the office, causing no pollution is highly important.
From these standpoints, there have been proposed a-Si series electrophotographic light receiving members comprising an amorphous silicon hydride material (hereinafter referred to as a-Si:H), for example, as disclosed in U.S. Pat. No. 4,265,991 which discloses an electrophotographic light receiving having a photoconductive layer constituted by an a-Si:H material.
These electrophotographic light receiving members can be produced by a manner of heating to and maintaining an electrically conductive substrate at a temperature of 50 to 400xc2x0 C., and forming a light receiving layer comprising an a-Si material by means of an appropriate film-forming process such as vacuum evaporation, sputtering, ion plating, thermal CVD, photo-assisted CVD, or plasma CVD.
Of these, the film-forming process by means of plasma CVD in which direct current (D.C.) discharge, high frequency dischage or microwave glow discharge is generated in a film-forming raw material gas to decompose the film-forming raw material gas whereby forming an a-Si deposited film on a substrate maintained at a desired temperature has been evaluated as being the most appropriate, and it has been using in practice.
Besides, U.S. Pat. No. 5,382,487 discloses an electrophotographic light receiving member comprising a light receiving layer (a photoconductive layer) comprised of an a-Si material containing halogen atoms (X) (this material will be hereinafter referred to as a-Si:X) formed on an electrically conductive substrate. In this document, it is described that by incorporating halogen atoms in an amount of 1 to 40 atomic % into an a-Si material, there can be attained a light receiving layer which is highly heat resistant and has good electric and optical properties required for a photoconductive layer in an electrophotographic light receiving member.
Japanese Unexamined Patent Publication No. 57-115556 discloses a technique for improving a photoconductive member having a photoconductive layer comprising an a-Si deposited film with respect to its electric, optical and photoconductive properties including dark resistance, photosensitivity, and photoresponsibility and also with respect to its use environmental characteristics including moisture resistance, by disposing a surface barrier layer constituted by a non-photoconductive amorphous material containing silicon and carbon atoms on a photoconductive layer constituted by an amorphous material containing silicon atoms as a matrix.
Japanese Unexamined Patent Publication No. 60-67951 discloses a photosensitive member having a light transmissive and insulative overcoat layer comprising an amorphous silicon material containing carbon atoms, oxygen atoms and fluorine atoms. Japanese Unexamined Patent Publication No. 62-168161 discloses a light receiving member having a surface layer constituted by an amorphous material containing silicon atoms, carbon atoms and hydrogen atoms in an amount of 41 to 70 atomic %.
Furthermore, Japanese Unexamined Patent Publication No. 60-95551 discloses a technique for improving the quality of an image reproduced by an amorphous silicon electrophotographic light receiving member by conducting an image-forming process including charging, exposure, development and transfer steps while maintaining the temperature in the vicinity of the surface of the light receiving member in the range of 30 to 40xc2x0 C., whereby a reduction in the surface electrical resistance of the surface of the light receiving member which will be caused when moisture is deposited on the surface and occurrence of a smeared image in an image reproduced because of the reduction of the surface electrical resistance are prevented.
In addition, Japanese Unexamined Patent Publication No. 61-283116 discloses a microwave plasma CVD (hereinafter referred to as MW-PCVD) process and a MW-PCVD apparatus which are suitable for the production of an amorphous semiconductor. Japanese Unexamined Patent Publication No. 63-149381 discloses a manner in which a plurality of cylindrical substrates are concentrically arranged at a desired interval so as to establish a discharge space circumscribed by the cylindrical substrates in a deposition chamber, and a microwave power is introduced into the discharge space, whereby an amorphous film is formed on each cylindrical substrate.
Based on the foregoing knowledges, it is possible to realize a desirable a-Si series electrophotographic light receiving constituted by an a-Si material, which is satisfactory in electric, optical and photoconductive characteristics, use-environmental characteristics and durability, and enables to reproduce a high quality image.
In the following, description will be made of an example of an apparatus and a process for producing such a-Si series electrophotographic light receiving member.
FIG. 3 is a schematic diagram illustrating an example of an apparatus for the production of an electrophotographic light receiving member by means of an RF-plasma CVD process using a frequency belonging to the RF band (this apparatus will be hereinafter referred to as RF-PCVD apparatus).
The RF-PCVD apparatus shown in FIG. 3 roughly comprises a deposition apparatus 3100, a raw material gas supply system 3200, and an exhaust system for evacuating the inside of a reaction chamber 3111 in the deposition apparatus 3100.
In the reaction chamber 3111 of the deposition apparatus 3100, there is provided a cylindrical substrate holder 3113 for holding a cylindrical substrate 3112 (on which a film is to be formed) positioned thereon. The cylindrical substrate holder 3113 is provided with a heater 3113xe2x80x2 installed therein, which serves to heat the cylindrical substrate 3112. Reference numeral 3114 indicates a raw material gas feed pipe (usually having a plurality of gas release holes capable of uniformly supplying a raw material gas toward the cylindrical substrate 3112) which is provided in the reaction chamber 3111.
The raw material gas feed pipe 3114 is communicated with the raw material gas supply system 3200 through a gas piping 3116 provided with a sub-valve 3260.
Reference numeral 3115 indicates a high frequency power matching box extending from an RF power source (not shown), which is electrically coupled to the deposition chamber 3111. Each of reference numerals 3120 and 3121 indicates an insulating member which is provided at the circumferential wall of the reaction chamber 3111.
The reaction chamber 3111 is provided with an exhaust pipe provided with a main valve 3118, which is connected to an exhaust device (not shown). Reference numeral 3117 indicates a leak valve, and reference numeral 3119 a vacuum gage.
The raw material gas supply system 3200 comprises gas reservoirs 3221-3226 for raw material gases such as SiH4, GeH4, CH4, H2, B2H6, PH3, and the like; and valves 3231-3236, inlet valves 3241-3246, exit valves 3251-3256 and mass flow controllers 3211-3216 respectively corresponding one of the gas reservoirs 3221-3226. The raw material gas supply system 2200 is designed such that the raw material gas from each gas reservoir can be introduced into the reaction chamber 3111 through the sub-valve 3260 of the gas piping 3116, and the raw material gas feed pipe 3114.
The formation of a deposited film using the RF-PCVD apparatus shown in FIG. 3 may be conducted, for example, in the following manner.
A cylindrical substrate 3112 is positioned on the substrate holder 3113 in the reaction chamber 3111. The inside of the reaction chamber 3111 is evacuated to a desired vacuum degree by means of the exhaust device comprising a vacuum pump for example (not shown in the figure). The temperature of the cylindrical substrate is controlled to a desired temperature in the range of 20 to 450xc2x0 C. by means of the heater 3113.
Particularly, prior to the entrance of raw material gases into the reaction chamber 3111, it is confirmed that the valves 3231-3236 for the gas reservoirs 3221-3226 and the leak valve 3117 of the reaction chamber 3111 are closed and that the inlet valves 3241-3246, the exit valves 3251-3256, and the sub-valve 3260 are opened. Then, the main valve 3118 is first opened to evacuate the inside of the reaction chamber 3111 and the inside of the gas piping 3116 by the vacuum pump (not shown).
Then, upon observing that the reading on the vacuum gage 3119 becomes a predetermined vacuum degree of, for example, about 5xc3x9710xe2x88x926 Torr, the sub-valve 3260 and the exit valves 3251-3256 are closed. Thereafter, the valves 3231-3236 are opened to introduce raw material gases from the gas reservoirs 3221-3226, and the pressures of the respective gases are adjusted to 2 Kg/cm by means of the pressure controllers 3261-3266. Then, the inlet valves 3241-3246 are gradually opened to introduce the respective gases in the mass flow controllers 3211-3216.
After completing the preparation for the film formation as described above, the formation of a deposited film as a light receiving layer is conducted as follows.
After the temperature of the cylindrical substrate 3112 becomes stable at a desired temperature, one or more of the exit valves 3251-3256 (which are necessary to be used for the formation of the deposited film) and the sub-valve 3260 are gradually opened to introduce one or more given raw material gases (which are required for the formation of the deposited film) from one or more of the gas reservoirs 3221-3226 into the reaction chamber 3111 through the gas feed pipe 3114. The flow rate of each raw material gas is controlled to a predetermined value by means of one or more of the mass flow controllers 3211-3216 involved. In this case, the gas pressure (inner pressure) of the reaction chamber 3111 is adjusted to a predetermined value of less than 1 Torr by regulating the opening of the main valve 3118 while observing the reading on the vacuum gage 3119.
After all the flow rates of the raw material gases and the inner pressure of the reaction chamber 2111 becomes stable, a high frequency power (having an oscillation frequency of 13.56 MHz) of a desires wattage from a high frequency power source (not shown) is applied into the reaction chamber 3111 through the matching box 3115 to cause glow discharge in the raw material gases introduced therein, where the raw material gases are decomposed to case the formation of a deposited film containing, for example, silicon atoms as a matrix on the cylindrical substrate 3112.
After the deposited film is formed at a desired thickness on the cylindrical substrate 3112, the application of the high frequency power is suspended and the exit valves are closed to suspend the introduction of the raw material gases into the reaction chamber. By this, the formation of the deposited film as the light receiving layer is completed.
If necessary, by repeating the above film-forming procedures several times, there can be formed a light receiving layer having a multi-layered structure. In this case, all the exit valves other than those required for forming the respective layers are of course closed.
Further, if necessary, upon forming the respective layers, the inside of the system is once evacuated to a high vacuum degree as required by closing the exit valves 3251-3256 while opening the sub-valve 3260 and fully opening the main valve 3118 in order to prevent the gases used for the formation of the previous layer to be left in the reaction chamber 3111 and also in the gas pipe ways.
In this way, there can be formed a desired deposited film such as an a-Si deposited film as a light receiving layer on a cylindrical substrate. By this, there can be produced an electrophotographic light receiving member (or an electrophotographic light receiving drum).
In the above, in order to improve the uniformity of the deposited film formed, it is possible to rotate the cylindrical substrate 3112 at a desired speed during the formation thereof by means of a deriving means (not shown). Further, in the above, it is a matter of course that the raw material gases used and the valve operations are properly changed depending upon conditions for forming a desired deposited film.
Other than the foregoing RF-PCVD apparatus, there is known a film-forming apparatus as shown in FIGS. 4(A) and 4(B), which enables to produce a plurality of electrophotographic light receiving members at the same time.
Particularly, FIG. 4(A) is a schematic diagram illustrating a film-forming apparatus by means of a microwave plasma CVD process (this apparatus will be hereinafter referred to as MW-PCVD apparatus), which enables to mass-produce an electrophotographic light receiving member. FIG. 4(B) is a schematic cross-sectional view, taken along the line B-Bxe2x80x2 in FIG. 4(A).
The MW-PCVD apparatus shown in FIGS. 4(A) and 4(B) comprises a reaction chamber 401 having a center longitudinal axis and which is provided with an exhaust pipe 404 connected through an exhaust valve (not shown) to an exhaust device (not shown).
The reaction chamber 401 has opposite end portions, each of which being hermetically provided with a microwave transmissive window 402 to which a waveguide 403 extending from a microwave power source (not shown) is connected. In the reaction chamber 401, six cylindrical substrate holders 407 each having a cylindrical substrate 405 (on which a film is to be formed) are concentrically and spacedly arranged in substantially parallel to each other so as to circumscribe the center longitudinal axis. Each cylindrical substrate holder 407 is provided with a heater 407xe2x80x2 installed therein, which serves to heat the cylindrical substrate positioned thereon from the inner side. Each cylindrical substrate holder 407 is held on a rotary shaft 408 connected to a driving mechanism comprising a reduction gear 410 and a driving motor 409 so that when the driving motor 409 is actuated, the rotary shaft 408 is rotated through the reduction gear 410, whereby the cylindrical substrate positioned on the cylindrical substrate holder is rotated about the central axis in the generatrix direction.
Reference numeral 406 indicates a discharge space (or a film-forming space) circumscribed by the six cylindrical substrate holders 407 each having the cylindrical substrate 405 positioned thereon and the opposite microwave transmissive windows 402.
The reaction chamber 401 is provided with three longitudinal gas feed pipes 451 each being arranged between given adjacent two cylindrical substrate holders. Each gas feed pipe 451 is provided a plurality of gas release nozzles capable of uniformly supplying a raw material gas into the discharge space 406. All the gas feed pipes 451 are communicated with a raw material gas supply system including a plurality of gas reservoirs (not shown).
The production of an electrophotographic light receiving member using the above MW-PCVD apparatus is conducted, for example, as will be described below.
The inside of the reaction chamber 401 is evacuated to a vacuum degree of less than 1xc3x9710xe2x88x927 Torr by means of the exhaust device (not shown), followed by heating the cylindrical substrates 405 to and maintaining at a desired temperature by means of the heater 407xe2x80x2. Then, given film-forming raw material gas is introduced into the discharge space 406 of the reaction chamber 401, and simultaneously with this, a microwave power with an oscillation frequency of more than 500 MHz, preferably 2.45 GHz, is introduced into the discharge space 406 through the waveguide 403 and the microwave transmissive window 402, where glow discharge is generated in the discharge space containing the raw material gas therein to decompose and excite the raw material gas whereby causing the formation of a deposited film as a light receiving layer on each of the cylindrical substrates 405. In this case, by actuating the driving motor 409 as above described, the deposited film can be uniformly formed over the entire surface of each of the cylindrical substrates 405. By this, there can be produced six electrophotographic light receiving members at the same time.
According to the foregoing apparatus and processes, it is possible to produce a good a-Si series light receiving member for use in electrophotography. It is necessary to have a further technology progress in order to meet high level demands on the market. These demands include demands for further diminishing the occurrence of a defective image, the occurrence of a ghost based on the so-called photomemory phenomenon on an image reproduced, and the production cost.
The photomemory phenomenon herein means a phenomenon that a latent image formed in a given electrophotographic image formation cycle is somewhat remained without completely extinguished until the successive electrophotographic image formation cycle, and the residual latent image appears in the form of a ghost image on an image reproduced in the successive electrophotographic image formation cycle.
In order to attain a desirable a-Si series electrophotographic light receiving member which can meet such demands as above described, it is essential to develop a further progress in the technique for the production of an a-Si series electrophotographic light receiving member, particularly, in the technique for the formation of an a-Si film.
With respect to the technique for the formation of an a-Si film, in terms of applying the a-Si film in various uses including light receiving members for use in electrophotography, various contrivances and improvements have been made and as a result, the technique has been progressing.
For instance, as one of the techniques on which the public attention has been focused in recent years, there is a VHF-plasma CVD method in which a high frequency power with an oscillation frequency belonging to a VHF band is used.
This VHF-plasma CVD method has advantages such that the film deposition rate is high and a high quality a-Si film can be produced. In view of this, the VHF-plasma CVD process is expected so that it can achieve a desirable reduction in the production cost of a product and an a-Si film having a desirably improved quality.
And various studies have been made of various film forming processes in which the VHF-plasma CVD method is used.
However, in the case of producing an a-Si series light receiving member comprising a light receiving layer formed on a large area substrate for use in electrophotography using the VHF-plasma CVD method, a problem is liable to entail in that a high quality a-Si film as the light receiving layer is difficult to form at a uniform thickness over the entire surface of the large area substrate. The reason for this is considered such that when the VHF electrode used in the VHF-plasma CVD process is of a relatively small size, the film formed on a surface area of the substrate situated in the vicinity of the VHF electrode differs from that formed on a surface area of the substrate which is remote from the VHF electrode in terms of the film thickness and property, and on the other hand, when the VHF electrode is of a relatively large size, a variation is occurred in the density of a VHF power applied and this variation entails a variation for the film formed on the substrate in terms of the film thickness and property.
In order to prevent this situation from occurring, it is considered to use a plurality of small-sized VHF electrodes. However, this is not always effective for the reason that abnormal discharge is sometimes generated between adjacent VHF electrodes or discharge generated sometimes becomes unstable, and as a result, it is difficult to stably produce a desirable light receiving member.
In any case, in the case of producing a plurality of light receiving members particularly for use in electrophotography having a large area and relatively thick light receiving layer with a uniform property in a single film formation process, the plasma CVD apparatus and the plasma CVD process employed are necessary to be sufficiently optimized.
However, for the purpose of mass-producing a light receiving member at a high productivity, when a high frequency power in an excessive quantity is applied in order to form a light receiving layer on an increased number of large area substrates at a high deposition rate in a single film formation process, a problem is liable to entail in that the stability of discharge generated is decreased. When the stability of discharge generated is decreased, discharge discontinuance or spark (abnormal discharge) are liable to occur. When discharge discontinuance is occurred during film formation, the formation of a film deposited is tentatively suspended to cause the formation of an interface at a portion of the film for which the discharge discontinuance has been occurred. In order to resume discharge under this circumstance, it is necessary to supply a high frequency power which is greater than ordinary discharge-commencing high frequency power, where a structural defect is liable to occur at the interfacial portion. In the case where the foregoing spark should have been occurred during the formation of a deposited film, a portion of the deposited film for which the spark has been occurred is liable to be locally inferior in terms of the film property.
In the case where a light receiving member having a light receiving layer comprising the deposited film having such defective portion is used in an electrophotographic apparatus, when image formation by ordinary electrophotographic image-forming process is repeated, the durability thereof is insufficient and because of this, so-called ghost phenomenon in which an image once reproduced in a given electrophotographic image formation cycle remains in the form of a ghost image on an image reproduced in the successive electrophotographic image formation cycle is occasionally occurred. In the case where the electrophotographic image formation is continuously repeated over a long period of time, so-called blank exposure is irradiated to a portion of the light receiving member which is situated between a copying sheet used in a given electrophotographic image formation cycle and another copying sheet used in the successive electrophotographic image formation cycle, in order to prevent toner from being adhered onto said portion. In this case, it is liable to have such an occasion that a phenomenon in which the density of an image reproduced on the portion of the light receiving member having irradiated with the blank exposure is thinned (this phenomenon is remarkable in the reproduction of a halftone image), namely, a blank exposure memory, is apparently occurred.
Now, in the past, the electrophotographic image reproduction was mainly for characters and therefore, the request on the market for the quality of an image reproduced was not so high, where the foregoing problems were not so serious.
However, in recent years, photo images or minute images are often subjected to electrophotographic image reproduction and therefore, there is an increased demand for further improving the quality of an image reproduced, where the foregoing problems are serious. In this connection and also in view of industrial sake, it is an urgent necessity to establish an adequate film-forming apparatus and an adequate film-forming process which enable to mass-produce a desirable light receiving member for use in electrophotography which is free of the foregoing problems at a high yield.
An object of the present invention is to provide a plasma CVD apparatus and a plasma processing method which can solve the foregoing problems in the prior art and can form a high quality deposited film having a remarkably uniform film thickness and an uniform film quality on a plurality of cylindrical substrates not only in the axial direction but also in the circumferential direction of each cylindrical substrate at a high speed whereby enabling to efficiently mass-produce a high quality semiconductor device.
Another object of the present invention is to provide a plasma CVD apparatus and a plasma processing method which can efficiently mass-produce, at a high yield, a desirable light receiving member for use in electrophotography having a light receiving layer constituted by a non-single crystalline material particularly a non-single crystalline material containing silicon atoms as a matrix which always stably exhibits satisfactory electric, optical and photoconductive properties without depending on use-environments, is not deteriorated even upon repeated use over a long period of time and excels in durability.
A further object of the present invention is to provide a plasma CVD apparatus and a plasma processing method in which a high frequency power introduction means for introducing a high frequency power of a VHF band which comprises a plurality of electrodes and a dielectric arranged between each adjacent electrodes is used, and which can stably and efficiently mass-produce a large area light receiving member for use in electrophotography having a light receiving layer comprising an a-Si film having a remarkably uniform film thickness and an uniform film quality, whose occasion of suffering from a photomemory being markedly diminished over the entire area, and which is satisfactory in uniformity with respect to electrophotographic characteristics including charge retentivity and photosensitivity, while sufficiently preventing the occurrence of discharge discontinuance and abnormal discharge found in the prior art.
A further object of the present invention is to provide a plasma CVD apparatus comprising a reaction chamber capable of being substantially vacuumed, substrate holding means and a cathode electrode arranged in said reaction chamber, in which a high frequency power is supplied to the cathode electrode to generate plasma between a substrate held by the substrate holding means and the cathode electrode whereby processing said substrate by said plasma, wherein said cathode electrode comprises a plurality of electrically conductive members on the same axis which are capacitively coupled by a dielectric member.
A still further object of the present invention is to provide a plasma processing method comprising the steps of: introducing a raw material gas into a reaction chamber capable of being substantially vacuumed and applying a high frequency power to a cathode electrode positioned in said reaction chamber to generating plasma whereby processing a substrate held by substrate holding means in the reaction chamber, wherein a cathode electrode comprises a plurality of electrically conductive members on the same axis which are capacitively coupled by a dielectric member is used as the cathode electrode.